REGIONAL EVALUATION OF CUT-SLOPE INSTABIUTY USING STATISTICAL METHODS NORTHERN RANGE, TRINIDAD

REGIONAL EVALUATION OF CUT-SLOPE INSTABIUTY USING STATISTICAL METHODS NORTHERN RANGE, TRINIDAD

STANLEY RICH WHARTON
Ministry of Energy, Port-of-Spain,
Trinidad and Tobago

Cut-slope instability is a perennial problem along roads cut in weathered metamorphic rocks in the Northern range. The methodology for assessing the severity of these problems over a wide area, and using different types of variables, can be based on statistics. Statistics are used to estimate the parameters of the population and make decisions based on these estimates. The choice of statistical methods is determined by the data types and the kinds of decisions that are required.
One hundred slope units selected from ten roads across the Northern Range were used for an evaluation of factors that contribute to cut-slope instability. Standardised field data sheets were designed for a systematic collection of numeric and descriptive geological, geomorphological and geotechnical data from failed and unfailed slope sections. Parameters included the discontinuity orientation; the slope and slide morphology, orientation and size; gradation analysis, Atterberg limits residual soil; and the estimated mean annual rainfall zones for the wet and dry seasons. Descriptive data included the kind of upslope deforestation associated with slope failures, the location of failures at truncated topographic ridges, formational type, failure type and the degree of weathering.
Three main statistical tests were performed:
1. Cross-tabulation statistical test for alphanumeric, data; 2. One way ANOVA and Multiple Comparison tests for numeric data, and 3. three dimensional statistics for the discontinuity data. FLCL (failure class) and FMTP (formation type) were selected as grouping variables for two of the statistical test, which were performed at the 0.05 level of significance.
The results from the combined statistical tests indicated that:

1. The majority of cut-slope failures were located at truncated topographic ridges;

2. The occurrence of slope failure was specific to the layers within the weathering profile;

3. Soils from earth slides showed high liquid limits;

4. Upslope deforestation by human activity was a major contributor to cut-slope fail ures;

5. Slope failures were frequent in the high mean annual rainfall zones;

6. Rock slides and debris slides involved, the largest volumes of material and were frequent in the Grande Riviere and Maracas Formations; the Grande Riviere Formation was more failure prone than the Maracas and Rio Seco Formations;

7. The critical discontinuities showed a strong preferred trend towards the south-east and the north-west.

The technique provides a novel approach to regional cut slope stability evaluation. The results may be useful forthe development of a framework for land use planning, the prediction and prevention of cut-slope failures, and the provision of basic information for site-specific studies on cut-slope failure.

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